Production of crystals



L. PESSEL Filed Nov. 1, 1945 PRODUCTION OF CRYSTALS Aug. 17, 194s.

Patented 17, 1948 amata raonUc'noN. or cars'rns Leopold remi,Philadelphia. rs., mlm to Radio Corporation of America, a corporation ofDelaware Application November 1, 1945,-Serlal No. 626,135

23 Claims. (CL 171-327) 'Ihis invention relates to the art' of producingcrystals and 'more specifically to producing synthetically, crystalswhich exhibit a piezo-electric effect.

One obj ect-of this invention to produce monol crystal bodies. invgeneral, synthetically by re,-

crystallization of finely subdivided crystalllzable substances. f

Another object is to produce synthetic oscillator plates of quartz orother piezoelectric substances.

Another object is to change the frequency of existing oscillator platesby synthetic crystal growth.

A further object of the invention is to produce synthetic crystals insuch a shape and form that a minimum -of additional mechanicaloperations has to be performed to produce a. finished oscillator plate.

Another object is to produce a large number of oscillator platessimultaneously, with little frequency deviation between them.

occurring therein crystals of suitable quality. Only a very smallpercentage of the mined mineral is sent to the laboratories where thecrystals are cut and shaped. Yet, suchi-s the uncertainty of prevailingtests which can be used in the field,

' that of the supposedly perfect crystals finallyv sent to thelaboratory more than half must be I ,discarded when the oscillatorplates are being cut.

' Another source of piezoelectric crystals has been the well knownRochelle salt or sodium potassium tartrate. Here the prevailing methodshave been to grow crystals from the melt or from solution. Rochelle saltcrystals, however, have proven unsuitable for use at high frequencies`and have mechanical limitations such as softness and highy solubility.

Certain methods similar to those used in making Rochelle salt crystalshave been tried in making suitable quartz crystals but the results havebeen disappointing since the product is inferior in both size andquality to the crystals which occur naturally.

` out the world for natural quartz deposits having 2 The presentinvention represents a distinct change from any of the above describedmethods and can be applied not only to quartz but to tourmaline,Rochelle salt, and, in fact,to the forH mation of any crystallinematerial.

In general, the finely subdivided substance, for example, powderedquartz or silica, is mixed with a, small proportion of a solvent. Acrystallization catalyst may be added, if speed of reaction is desired.In the case of quartz this catalyst may -be a substance which gives analkaline reaction such as the hydroxides of the alkali and alkalineearth metals as well as some of the stronger ory ganlc bases. Othertypes of catalyst which have been found useful are the inorganicfluoride salts and hydrouoric. acid. Although these catalysts arepreferred in the case of quartz the invention contemplates the use, ingeneral, of any substance which catalyzes the crystallization of thesub? stance the crystal of which is being formed. The mixture iscompressed into a substantially plateshaped body and maintained atelevated pressure and preferably at elevated temperature. The compressedbody i-s subjected to high-frequency oscillations by means of suitablyspaced electrodes. One frequency component is preferably the naturalmechanical vibration frequency of the compressed body. A supersonicfrequency component may -be added. It is also possible to use other high.frequencies of alternation which will produce the desired vibrations inthe compressed yamorphous material. Novel features are shown for causingsuch crystallization to take place and to proceed in such an orientedmanner thatthe ultimate result is a body` showing the characteristics ofa 4monocrystal.'

The invention will now be described more specifically as it relates toquartz bearing in mind,

howevernthat the same general principles apply in the case of othersubstances.

It is Well known that quartz or silica isvery slightly soluble in water.This solubility increases when the material is powdered and thisincrease depend-s 'upon the degree of subdivision of the particles. Inaddition, the solubility is increased by raising the .temperature andpressure. Further increases in solubility are caused by the presence ofother substances. Some of those substances which have been foundparticularly useful in practicing the invention are the alkalies inwhich themetal is of the class of alkali or alkaline earth metals, aswell as strong organic Other classes of substances found usefulbases.are the fluoride salts and hydroiluoric acid.

lThe material used should be extremelysubdivided silica','such aspowdered quartz.'ch alcedony.'" The para..v ticles are `preferably of 10micron diameter 0I although coarser particles may be hydrated silica,oramorphous silica.

pressure used may be made use of.

' I'he powdered silica is mixed with a vsma amount of water which amountis not ilxed but Vdepends -upon particle size and shape as well-as uponother operating conditions. To this mixture is preferably added a smallamount of one of the catalysts heretofore enumerated in order Theprepared mixture is then treated by the method presently to be describedusing suitable apparatus which is diagrammatically represented in thevarious figures, of which:

Fig. 1 represents diagrammatically a longitudinal section view ofapparatus suitable for apply ing a single high frequency component. z

Fig. 2 represents a modification in which an additional supersoniccomponent is added at right angles to the high frequencycomponent.

' Fig. 3 represents a modification in lwhich the v high frequencyelectrodes are placed at an' angle to theplane of the faces of thechamber. -v

Fig. 4 represents a modification in which the high frequency electrodesare placed parallelto the main faces of the oscillator plate buthaving[the electrodes of the supersonic component at -some angle other than aright angle inrelat'ion.

' to the'highfrequency electrodes. v l v Fig. 5 shows a modiilcationin`.whichgthefce'nters of thehigh frequency electrodes are "laterally-5displaced.

' Fig..6 shows a variation of the abovednwhich theelectrodes of thesupersonwboinponent are 1 also atan angle other than afrightfangleto the"planesofthe high frequencyfelectrodes'-j. Fig. .7.1 shows amodification' irl-which quartz.

oscillator plates vare used in place of metalfelectrodes. f

.Where it is not necessary or desirable to speed up the solutionprocess, ordinary room temperature may beused. As in the case of thecatalyst,v

increased temperature is used only to speed u'p the process and-may beomitted if desired.

Elevated pressure is then applied to top plate fl through shaft i5 inorder to compact the particles and accelerate the recrystallization rejaction. 'I'his pressure is preferably not less than the vapor pressureof water at the temperature where `=the lfundamental thickness-mode inFig. 8 represents a modification bylinea'ns' of which a quartz crystalof given frequency'may a thicker crystal ofA lower free be changed intolquency. Y l

Fig. 9 represents a modication in which more selected but may beconsiderably higher.

An electric ileld alternating at high frequency is maintained betweenelectrodes i and 2 which are insulated from each other. This highfrequency field may be supplied by any conventional type of highfrequency' oscillator such as shown diagrammatically atA. The electrodesare connected to the generator through conductors I 9.

Where only a single alternating field component is usedit is preferablethat said frequency correspond to vthe natural'vibration frequency ofthe Acrystal being formed. However, some other frequency suitable'forsetting up the necessary mechanical strains may be used. For quartz thisfrequency mayvary all the way from kilocycles' per second (50,000cycles/second) to 10 megacyclesper second (10,000,000 cycles/second) and"evenhigher.v

'Ifhe frequency with which a quartz plate oscil- 'latesis. `representedby the equation r ft" megacycles/second, t is -thickness in mils of aninchand K i's'ja constant, 99 for the V2 cut 'quartz crystal'and 66 forthe VI cut (see U. S.

patent; .to Gerber, 2,304,760). Since the frequency is 'inverselyproportional to the thickness,

the greater-the thickness the lower will be the .frequency of theoscillator plateproduced. The

frequency range Amentioned represents about the practical? limits ofcommercially usable quartz vvoscillator plates at the present time`although it is possible togetsome results-.with both higher thantwoelectrodes are used and in which the fields are parallel.

Fig. l0 represents a modification in' whichmore than two electrodes areused and in whi'ch the elds are not parallel. f Fig. l1 represents aview oi' right angles to the view shown in Fig. 1.

the' appara-.tus I. shown diagrammatically in Fig. 1 but vtaken kat''and catalyst added is packed inthe compartment; Y

as-shown in Fig. l. The mass is raised tolan "5 elevated temperaturewhich may be anywhere i'rom slightly above room temperature, as 50 C upto the inversion temperature of quartz (5'I 3 C.). A preferredtemperatureis the criti-y '1 cal point oi' water (374 CJ. 'I'hetemperature is controlled by resistance heaters I4 connected 'to' the A.C.mains by conductors 20.' Any suitable A. C. current controlling meansof conventional and lower -frequencies.

Although not wishing tolbe limited to the theory ,of what supposedlyoccurs during the above-described process it is thought to take place asfollows: l

When the mixture of finely divided silica,

water and catalyst is subjected to elevated temperature and pressure,some of the silica goes into solution. Since this reaction takes placein a-closed system'some of the silica first dissolves then redepositsinaccordance with the principles of fphase. equilibrium resulting in -a.gradual growth of the crystalline particles. This growth shouldtheoretically proceed until the whole mass `r`has been transformed intoone single crystal. 1 However, when*` nov -means are provided fororlenting tlie c'rystallites,` growth takes place in a haphazardfmanneras toV crystal orientation.

In addition-ae the crystallits grew there is a corresponding decrease inthe active reaction area. The result of these two factors, if allowed toproceed'without the conditions prescribedl by `the present invention,would be the'formation ot a crystalline aggregate showing extensivetwinning rather than formation of a monocrystal.

When the xnicrocrystallineA or amorphous powder is compressed, as in thepresent invention. to-such an extent that itbehaves like a rigid body,the possibility is given to direct the orientation of the growingparticles into uniform axial directions. It is well lfnown that crystalgrowth shows directional response to mechanical stresses. In the methoddescribed, such mechanical stresses are.gcreated by subjecting thecompressed mixture to one or more alternating electric fields.

There are various modifications of the method which may be used toadvantage some of which may be utilized to produce crystals of special'types.

In the modification shown in Fig. 2, there is used in addition to the,high frequency ileldI a source of supersonic vibrations B connectedthrough conductors 2| to end electrodes I8. The use of these supersonicvibrations aids in the mechanical reorientation of the particles andthus promotes oriented crystallization. The frequency range useful hereis 15-50 kilocycles/second.

In order to produce quartz oscillator plates of other than the X-cuttype other modifications of the method may be used. As in Fig. 3, thehigh frequency field `may be applied by electrodes 26, 21 whose surfacesare placed at some angle with respect to the main faces of theinsulating plates i2 and 25.

In Fig. 4 the high frequency electrodes I and 2 are placed as in Fig. 1but the electrodes Il ccnnected to the source B of supersonic vibrationsare placed at an angle to thehigh frequency electrodes. y

In order to provide for a high frequency field which will travel at anangle through the powdered mass, the electrodes may be arranged as at 3and l in Fig. 5. Here the high frequency electrodes are set in theinsulating plates I3 .with their centers laterally displaced. V

A further modification is shown in Fig. 6 in which the electrodesconnected to the source of supersonic vibrations are also set at anangle to the main faces of the oscillating plate being formed.

It has also been found desirable to sometimes set up two or more highfrequency fields in orienting thefcrystallites. Such an arrangement isshown in Fig. 9 in which the fields are parallel. Electrodes 'l and 8,when connected to high-frequency source A2 produce one field whileelectrodes 9 and I0 connected to high frequency source A1, produceanother field parallel to the first. The elds may have the same ordifferent frequencies.

In Fig. 10 the electrodes 9 and i0 produce a eld which is not parallelto that set up between electrodes 'l' and 8'.

The alternating mechanical stresses required for orientedrecrystallization are best produced, in piezoelectric substances, by analternating electric field. In other materials, however, such stressesmay be applied to the parent body by an `oscillating element orelements, such as quartz oscillator plates Vibrating at the properfrequencies which may, wholly or partly, replace the metal electrodespreviously described. This is illustrated in Fig. 7 where 5 and 6 arehooked up to high frequency oscillator C. Electrodes 21 and 28 are metalplates through which quartz plate 5 is caused to vibrate. Electrodes 29and asavgsea 6 lll perform a similar function for quarts plate l. Theinvention may alsobe used in buildingup the dimensions of existingquartz oscillator plates.

As illustrated in Fig. 8, a quartz oscillator plate 2l is packed in apowdered silica, water, catalyst mixture 22. Either a high frequencyfield only may be applied or a two component field of both highfrequency and supersonic vibrations may be used. In this way theoriginal crystal may be made larger.

The invention has been described mainly in connection with themanufacture of synthetic are met.

' These conditions are:

1. The material is placed in'a of subdivision.

\ 2. A liquid is added veryl fine state in which the finely dividedmaterial is but slightly or moderately soluble.

3. A crystallization catalyst is desirable although not absolutelynecessary.

4. Elevated temperature isused when accelerated reaction is desired.

. 5. Elevated pressure of sullieient degree is used to compress themixture so that it acts as a rigid body.

6. An alternating field of one or more components is used, at least oneof which is preferably the natural mechanical vibration frequency of theparent body.

.While in some cases it is desired to produce a l crystal which has mostof the sameproperties as one occurring naturally, it ls possible toproduce crystals which are distinctly different. For example, thesynthetic crystals may have different lattice constants than those ofnatural origin. This is possible because of the fact that the syntheticcrystals have been produced under mechanical strains not present in thekenvironment when the natural crystals were formed.

In the illustrations the top plate of the compartment is assumed to bemovablel to provide for exerting pressure on the mass andin all casesmeans are assumed for suitably heating the Kas-.-v

sembly where this is desired.

The method ofv heating the assembly is not by any meanslimited to theuse of resistance wireheating elements. For example, the vmetal elecbeheated by induction and the heat trodes may thus transferred to thepowdered mass. `Also it is feasible to use high frequency dielectricheating in which case in the powdered substance.

I claim as my inventlonz 1. The processV of producing a substantiallymonocrystalline body comprising mixing a .powder of a crystallizablesubstance with a liquid capable of bringing about partial solution ofsaid substance, compressing said mixture within a compartment,subjecting the compressed mixture to elevated temperature, andsubjecting the compressed, heated mixture to high-frequency vibrationsuntil substantially all of the mixture has crystallized into a solidbody.

2. The process of producing a substantially monocrystalline bodycomprising mixing a' powder of a crystallizable substance with a liquidcapable of bringing about partial solution of said f' substance to whicha catalyst of crystallization is added, compressing said mixture withina compartment, subjecting the compressed mixture to elevatedtemperature, and subjecting the compressed, heated mixture tohigh-frequency vibrathe heat is induced directly withv.

the mixture has vmixture to high-frequencyI vibrationsuntilsubstantially all of the mixture has crystallized into a solidbody. v 4. The invention as set forth in claim l, characterized by thehigh-frequency vibrations being applied through the medium of insulatedmetal electrodes. v

5. The invention as set forth in claim ,1, characterized by thehigh-frequency vibrations being applied through the medium ofpiezoelectric oscil-v lator plates.

6. The invention as set forth in claim 1, characterized by said mixturebeing compressed into a substantially piate-shapedbody and saidhighfrequency vibrations being set up by an alternat ing electric leldof a frequency corresponding to the natural mechanical vibrationfrequency of said body. f

7. The invention as set forth in claim 1, char- 4 acterized by saidmixture being compressed into asubstantially plate-shaped body and saidhighfrequency vibrations being set up by an alternating electric fieldone frequency component of which isd the natural mechanical vibrationfrequency of the compressed body and another frequency component ofwhich is in 'the supersonic range.

I acterized 'by more than `il. The invention as set forth in claim 1,.charn "acterized by the mixture being compressed into a substantiallyplate-shaped body, said highfrequency vibrations being set up by analternating electric field which is not vertical to the main plate facesof said body.

9. The invention as set forth in claim 1, char'- acterized by themixture-being compressed into a substantially plate-shaped body,saidbody beingvsubiected to alternatingjelectrlc elds, the fields beingin a direction not parallel to each other, one field alternating at thenatural mechanical vibration frequency of said body and another fieldalternating at a frequency in the supersonic range.

10. I'he invention as set forth in claim 1, characterized by thecrystallizable substance .being capable of forming piezoelectriccrystals.

11. The process of producing a quartz oscillator plate comprising mixingflnely'divlded silica with water. compressing the mixture into asubstantially plate-shaped body within a compartment, heating themixture to a temperature between 50 C. and 573 C. while maintaining apressure notlower than the vapor pressure of water at said temperature,and maintaining an alternatlng electric field across the body.

12. The process of claim 11, characterized by the water containing adissolved crystallization catalyst.

13. The process of claim 11, characterized by the water containing adissolved crystallization catalyst, said catalyst being a compoundwhich-is alkaline in nature.

, frequency corresponding 14. The process of claim 11, characterized bythe water containing a dissolved crystallization catalyst, said catalystbeing a compound selected from the group consisting of hydroiluoric acidand metallic uorides. 15. The process as set forth in claim 11,characterized by the alternating electric field being of a frequencycorresponding to the natural mechanical vibration frequency of thecompressed body.

16. The process as set forth in claim 11, characterized by saidalternating electric field having more than one component, one of whichis the natural mechanical vibration frequency of the compressed body andanother component of which is a frequency in the supersonic range.

17. The process as set forth in claim 11, charone type of alternatingfield being maintained across the compressed body. the direction ofwhich fields is not parallel to each other, 'and at least one of saiddirections being not vertical to the 'main faces of the plate, thefrequency of' one field being the natural mechanical vibration frequencyof the compressed body, the frequency .of another field being in thesupersonic range. y

18'. The process of superimposing oriented growth upon a monocrys'talbody comprising packing around said body ay mixture consisting of thefinely subdivided substance'of said body and a solvent in which saidsubstance is slightly soluble, exposing said Vcombination of body andmixture to elevated temperature and pressure and Amaintaining acrosssaid! combination an alternating electric eld.

19. The .invention as set forth in claim 18, characterized by the bodybeing a quartz plate oscillator.

5 20.-'.I'he process ,as set forth in claim 18, characterized by thealternating electric eld being of a frequency corresponding tothenatural vibration frequency'of the compressed combination.

21..;The invention-as set 'forth in claim 18, characterized byfsaid'alternating' field having morethan one component, one of which is of ato the natural mechanical vibration frequency oi said combination andanother of which is a frequency in the supersonic range.

22. A-y monocrystalline bodyf produced by oriented v'growtbofn'elydivid'ed crystallizable substance in`v the presence of a lsolventfor "said substanceA'under-the influence of elevatedtemperature,.pressure, .andan alternating electric field.

23. A monocrystalline body produced by oriented growth of finely dividedsilica in the presence of a small amount of a solvent for said substanceunder the inuence ofelevated temperature, pressure and an alternatingvelectric field.

' LEOPOLD PEssEL.

REFERENCES forrnn UNITED STATES PATENTS Name Date Meissner Nov. 1, 1932Number

